Synchronous motors are utilized in textile and glass inudstries and other applications requiring precise synchronization of multiple motors. Often these multiple motors are energized from variable frequency sources to provide high or low speed. In many prior art applications these multiple motor drives have used synchronous motors of the reluctance type such as in U.S. Pat. Nos. 3,126,493 or 3,652,885. These did eliminate the need for DC excitation power to the motor field, yet such motors had poor power factor and tended to be slightly unstable at low speeds, i.e., certain low frequency operating points.
The prior art synchronous permanent magnet motors have usually been constructed with either Alnico or Ferrite magents in the rotor. The Alnico magnets have a fairly high induction density but unfortunately have a very low coercive force so that the magnets are readily demagnetized by the M.M.F. from the primary winding on the stator of the motor. Conversely, Ferrite magnets have good coercive force but have low induction density. With such magnets, the rotor volume must be increased considerably to obtain acceptable power factor and horsepower rating. Thus the motor is large for its horsepower rating and uneconomical to manufacture as well as having an inherent lower maximum speed before disintegration. A motor of this type is illustrated in U.S. Pat. No. 3,492,520. Permanent magnet synchronous motors currently manufactured by some manufacturers are ones wherein the magnets are disposed in the rotor with the long dimension radially and magnetized circumferentially and thus two magnets act in parallel to supply the flux for a given pole on the rotor. This has the disadvantage that the shaft must be non-magnetic or must have a non-magnetic sleeve in order to avoid degradation of performance by leakage flux from one face of the magnet through the shaft and back to the other face of the magnet. Also leakage flux can have a path from one face of the magnet to the other through a magnetic bridge along the outer periphery of the rotor which further bleeds flux away from the useful flux crossing the air gap to the stator.
The problem to be solved therefore is how to construct a permanent magnet synchronous motor which has a small physical size, is economical to manufacture for its horsepower rating, will withstand high rotational speeds yet one which has high power factor and efficiency.